Oled pixel driving circuit and oled display device

ABSTRACT

The invention provides an OLED pixel driving circuit and OLED display device. The OLED pixel driving circuit uses 5T1C structure and switch (K). By switch signal (Switch) controlling the switch (K) to connect first pin (K 1 ) and second pin (K 2 ) to enter display mode, and to connect first pin (K 1 ) and third pin (K 3 ) to enter sense mode, the ADC (ADC) senses the threshold voltage of the fourth TFT (T 4 ), converted and used for data compensation in the display mode, able to compensate and eliminate the impact of threshold voltage of the fourth TFT (T 4 ) on the current flowing through the OLED (D 1 ); the light-emitting control signal (EM) controls the fifth TFT (T 5 ) connected to the OLED (D 1 ) to cut off during the data writing stage (t 1 ) to keep the data signal voltage and the gate voltage of the fourth TFT (T 4 ) unaffected by the OLED (D 1 ) threshold voltage.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to the field of display techniques, and inparticular to an OLED pixel driving circuit and an OLED display device.

2. The Related Arts

The organic light emitting diode (OLED) provides the advantages ofactive-luminescent, low driving voltage, high emission efficiency, quickresponse time, high resolution and contrast, near 180° viewing angle,wide operation temperature range, and capability to realize flexibledisplay and large-area full-color display, and is heralded as the mostpromising display technology.

The OLED is an electroluminescent device driven by electric current;that is, when a current flows through OLED, the OLED illuminates, andthe brightness is determined by the current flowing through the OLED.The majority of known integrated circuit (IC) only transmits the voltagesignal, and the pixel driving circuit of the OLED display mustaccomplish the task of translating the voltage signal into a currentsignal. The conventional pixel driving circuit usually uses a 2T1Cstructure, i.e., two thin film transistors (TFT) and a capacitor, totranslate the voltage into current.

As shown in FIG. 1, a conventional 2T1C pixel driving circuit fordriving OLED device comprises: a first TFT T10, second TFT T20, and acapacitor C10. The first TFT T10 is a switching TFT, the second TFT T20is a driving TFT, and the capacitor T10 is a storage capacitor.Specifically, the first TFT T10 has a gate connected to receive a scansignal Scan, a drain connected to receive a data signal Data, and asource electrically connected to a gate of the second TFT T20 and to oneend of the capacitor C10. The second TFT T20 has a drain connected toreceive a voltage OVDD of a power source, and a source connected toreceive an anode of the OLED D10; the OLED D10 has a cathode connectedto receive a common ground voltage OVSS; the capacitor C10 has one endelectrically connected to the gate of the second TFT T20, and the otherend electrically connected to the source of the second TFT T20. When theOLED displays, the scan signal Scan controls the first TFT T10 to beturned on, the data signal Data passes through the first TFT T10 andenters the gate of the second TFT T20 and the capacitor C10. Then, thefirst TFT T10 is cut off. Because of the storage of the capacitor C10,the gate voltage of the second TFT T20 stays at the data signal voltagelevel so that the second TFT T20 stays turned on. The driving currentflows through the second TFT T20 to enter the OLED D10 and drives theOLED D10 to emit light.

According to the equation calculating the current flowing through thedriving TFT and the OLED:

I _(OLED) =K×(Vgs−Vth)²;

Wherein I_(OLED) is the current flowing through the driving TFT and theOLED, K is the intrinsic conductive factor, of the driving TFT, Vgs isthe voltage difference across the gate and the source of the drivingTFT, and Vth is the threshold voltage of the driving TFT. As seen, thesize of I_(OLED) is related to the threshold voltage Vth of the drivingTFT.

The above conventional OLED pixel driving circuit is simple instructure, and without a compensation function, and therefore has manyshortcomings. The most prominent shortcoming is that the non-uniformityof the TFT fabrication process, the threshold voltages of the drivingTFTs of all pixels in the OLED display device will be inconsistent.Moreover, due to long operation time, the ageing of the driving TFT willcause voltage drift of the threshold voltage of the driving TFTs,leading to display unevenness.

The known technologies also include some OLED pixel driving circuit ableto sense the threshold voltage of the driving pixel and compensate thethreshold voltage to the data signal. However, the voltage of the datasignal is affected by the threshold voltage of the OLED. On one hand,the threshold voltage of the OLED must be about 10V, and thelight-emitting layer in the OLED must adopt the 3-layered or 4-layeredstructure. On the other hand, the data signal voltage cannot be higherthan the OLED threshold voltage during the data writing stage to ensurethat the OLED will not emit light during the data writing stage.Moreover, the gate voltage of the driving TFT is also affected by theOLED threshold voltage and is required to have a voltage level notgreater than the sum of the OLED threshold voltage and the driving TFTthreshold voltage.

SUMMARY OF THE INVENTION

The object of the present invention is to provide an OLED pixel drivingcircuit, not only able to compensate, but also to eliminate the impactof the threshold voltage of the driving TFT on the current flowingthrough the OLED, and achieve uniform display and improve image quality,as well as, keep the data signal voltage and the gate voltage of thedriving TFT unaffected by the OLED threshold voltage.

Another object of the present invention is to provide an OLED displaydevice, with pixel driving circuit able to compensate, improve displayuniformity, and keep the data signal voltage and the gate voltage of thedriving TFT unaffected by the OLED threshold voltage.

To achieve the above object, the present invention provides an OLEDpixel driving circuit, comprising: a first thin film transistor (TFT), asecond TFT, a third TFT, a fourth TFT, a fifth TFT, a capacitor, anOLED, and a switch, a DAC and an ADC disposed in a driving IC, operationstates of the OLED pixel driving circuit comprising a display mode and asense mode;

the switch being controlled by a switch signal, comprising a first pin,a second pin, and a third pin;

the first TFT having a gate connected to a scan signal, a drainconnected to a power source, and a source connected to a drain of thesecond TFT, a gate of the fourth TFT, and an end of the capacitor; thesecond TFT having a gate connected to the scan signal, and a sourceconnected to a common ground voltage; the fourth TFT having a drainconnected to a power source voltage, and a source connected to a drainof the fifth TFT; the fifth TFT having a gate connected to alight-emitting control signal and a source connected to an anode of theOLED; the OLED having a cathode connected to the common ground voltage;the capacitor having the other end connected to the source of the fourthTFT; the third TFT having a gate connected to the scan signal in thedisplay mode and connected to the sensing control signal in the sensemode, a source connected to the source of the fourth TFT, and a drainconnected to the first pin of the switch;

the switch having the second pin connected to the DAC, and the third pinconnected to the ADC;

in the display mode, the switch signal controlling the switch to connectthe first pin and the second pin, and the DAC providing the data signal;the display mode being divided into a data writing stage and alight-emitting stage, and the light-emitting control signal is at lowvoltage to cut off the fifth TFT during the data writing stage, and athigh voltage to turn on the fifth TFT during the light-emitting stage;in the sense mode, the DAC first providing a low voltage signal, andthen the switch signal controlling the switch to connect the first pinand the third pin so that the ADC sensing a threshold voltage of thefourth TFT.

According to a preferred embodiment of the present invention, in thedata writing stage of the display mode, the scan signal is at highvoltage, the common ground voltage is at low voltage, and the datasignal is at high voltage; in the light-emitting stage of the displaymode, the scan signal is at low voltage, the common ground voltage is atlow voltage, and the data signal is at high voltage;

in the sense mode, the scan signal first provides a high voltage pulseand then maintains at low voltage; the sensing control signal firstprovides a high voltage pulse synchronized with the high voltage pulseof the scan signal and then maintains at low voltage; the common groundvoltage always stays at low voltage; and the light-emitting controlsignal always stays at low voltage.

According to a preferred embodiment of the present invention, aresistance of the first TFT and a resistance of the second TFT have aratio relation.

According to a preferred embodiment of the present invention, thedriving IC presets a lowest grayscales to correspond to a highestvoltage of the data signal, and a highest grayscales to correspond to alowest voltage of the data signal; by adjusting the ratio between theresistance of the first TFT and the resistance of the second TFT, thegate of the fourth TFT has a voltage equal to the sum of the maximum ofthe data signal voltage and the threshold voltage of the fourth TFT.

The present invention also provides an OLED display device, whichcomprises an OLED pixel driving circuit; the OLED pixel driving circuitfurther comprising: a first thin film transistor (TFT), a second TFT, athird TFT, a fourth TFT, a fifth TFT, a capacitor, an OLED, and aswitch, a DAC and an ADC disposed in a driving IC, operation states ofthe OLED pixel driving circuit comprising a display mode and a sensemode;

the switch being controlled by a switch signal, comprising a first pin,a second pin, and a third pin;

the first TFT having a gate connected to a scan signal, a drainconnected to a power source, and a source connected to a drain of thesecond TFT, a gate of the fourth TFT, and an end of the capacitor; thesecond TFT having a gate connected to the scan signal, and a sourceconnected to a common ground voltage; the fourth TFT having a drainconnected to a power source voltage, and a source connected to a drainof the fifth TFT; the fifth TFT having a gate connected to alight-emitting control signal and a source connected to an anode of theOLED; the OLED having a cathode connected to the common ground voltage;the capacitor having the other end connected to the source of the fourthTFT; the third TFT having a gate connected to the scan signal in thedisplay mode and connected to the sensing control signal in the sensemode, a source connected to the source of the fourth TFT, and a drainconnected to the first pin of the switch;

the switch having the second pin connected to the DAC, and the third pinconnected to the ADC;

in the display mode, the switch signal controlling the switch to connectthe first pin and the second pin, and the DAC providing the data signal;the display mode being divided into a data writing stage and alight-emitting stage, and the light-emitting control signal is at lowvoltage to cut off the fifth TFT during the data writing stage, and athigh voltage to turn on the fifth TFT during the light-emitting stage;in the sense mode, the DAC first providing a low voltage signal, andthen the switch signal controlling the switch to connect the first pinand the third pin so that the ADC sensing a threshold voltage of thefourth TFT.

According to a preferred embodiment of the present invention, in thedata writing stage of the display mode, the scan signal is at highvoltage, the common ground voltage is at low voltage, and the datasignal is at high voltage; in the light-emitting stage of the displaymode, the scan signal is at low voltage, the common ground voltage is atlow voltage, and the data signal is at high voltage;

in the sense mode, the scan signal first provides a high voltage pulseand then maintains at low voltage; the sensing control signal firstprovides a high voltage pulse synchronized with the high voltage pulseof the scan signal and then maintains at low voltage; the common groundvoltage always stays at low voltage; and the light-emitting controlsignal always stays at low voltage.

According to a preferred embodiment of the present invention, aresistance of the first TFT and a resistance of the second TFT have aratio relation.

According to a preferred embodiment of the present invention, thedriving IC presets a lowest grayscales to correspond to a highestvoltage of the data signal, and a highest grayscales to correspond to alowest voltage of the data signal; by adjusting the ratio between theresistance of the first TFT and the resistance of the second TFT, thegate of the fourth TFT has a voltage equal to the sum of the maximum ofthe data signal voltage and the threshold voltage of the fourth TFT.

The present invention further provides an OLED pixel driving circuit,which comprises: a first thin film transistor (TFT), a second TFT, athird TFT, a fourth TFT, a fifth TFT, a capacitor, an OLED, and aswitch, a DAC and an ADC disposed in a driving IC, operation states ofthe OLED pixel driving circuit comprising a display mode and a sensemode;

the switch being controlled by a switch signal, comprising a first pin,a second pin, and a third pin;

the first TFT having a gate connected to a scan signal, a drainconnected to a power source, and a source connected to a drain of thesecond TFT, a gate of the fourth TFT, and an end of the capacitor; thesecond TFT having a gate connected to the scan signal, and a sourceconnected to a common ground voltage; the fourth TFT having a drainconnected to a power source voltage, and a source connected to a drainof the fifth TFT; the fifth TFT having a gate connected to alight-emitting control signal and a source connected to an anode of theOLED; the OLED having a cathode connected to the common ground voltage;the capacitor having the other end connected to the source of the fourthTFT; the third TFT having a gate connected to the scan signal in thedisplay mode and connected to the sensing control signal in the sensemode, a source connected to the source of the fourth TFT, and a drainconnected to the first pin of the switch;

the switch having the second pin connected to the DAC, and the third pinconnected to the ADC;

in the display mode, the switch signal controlling the switch to connectthe first pin and the second pin, and the DAC providing the data signal;the display mode being divided into a data writing stage and alight-emitting stage, and the light-emitting control signal is at lowvoltage to cut off the fifth TFT during the data writing stage, and athigh voltage to turn on the fifth TFT during the light-emitting stage;in the sense mode, the DAC first providing a low voltage signal, andthen the switch signal controlling the switch to connect the first pinand the third pin so that the ADC sensing a threshold voltage of thefourth TFT;

wherein in the data writing stage of the display mode, the scan signalbeing at high voltage, the common ground voltage being at low voltage,and the data signal being at high voltage; in the light-emitting stageof the display mode, the scan signal being at low voltage, the commonground voltage being at low voltage, and the data signal being at highvoltage;

in the sense mode, the scan signal first providing a high voltage pulseand then maintaining at low voltage; the sensing control signal firstproviding a high voltage pulse synchronized with the high voltage pulseof the scan signal and then maintaining at low voltage; the commonground voltage always staying at low voltage; and the light-emittingcontrol signal always staying at low voltage;

wherein a resistance of the first TFT and a resistance of the second TFThaving a ratio relation;

wherein the driving IC presetting a lowest grayscales to correspond to ahighest voltage of the data signal, and a highest grayscales tocorrespond to a lowest voltage of the data signal; by adjusting theratio between the resistance of the first TFT and the resistance of thesecond TFT, the gate of the fourth TFT having a voltage equal to the sumof the maximum of the data signal voltage and the threshold voltage ofthe fourth TFT.

Compared to the known techniques, the present invention provides thefollowing advantages. The present invention provides an OLED pixeldriving circuit with a 5T1C structure and using a switch. By the switchsignal controlling the switch, the first pin and the second pin areconnected to enter the display mode, and by the switch signalcontrolling the switch, the first pin and the third pin are connected toenter the sense mode, so that the ADC senses the threshold voltage ofthe fourth TFT, converted by ADC and used for data compensation in thedisplay mode. As such, the invention is able to compensate and toeliminate the impact of the threshold voltage of the driving TFT on thecurrent flowing through the OLED to achieve uniform display; thelight-emitting control signal controls the fifth TFT connected to theOLED to cut off during the data writing stage to keep the data signalvoltage and the gate voltage of the driving TFT unaffected by the OLEDthreshold voltage. The invention also provides an OLED display device,comprising the OLED pixel driving circuit, able to compensate improvedisplay uniformity and keep the data signal voltage and the gate voltageof the driving TFT unaffected by the OLED threshold voltage.

BRIEF DESCRIPTION OF THE DRAWINGS

To make the technical solution of the embodiments according to thepresent invention, a brief description of the drawings that arenecessary for the illustration of the embodiments will be given asfollows. Apparently, the drawings described below show only exampleembodiments of the present invention and for those having ordinaryskills in the art, other drawings may be easily obtained from thesedrawings without paying any creative effort. In the drawings:

FIG. 1 is a schematic view showing a conventional OLED pixel drivingcircuit with 2T1C structure;

FIG. 2 is a schematic view showing an OLED pixel driving circuitprovided by an embodiment of the present invention;

FIG. 3 is a schematic view showing the circuit connection in the displaymode of the OLED pixel driving circuit provided by an embodiment of thepresent invention;

FIG. 4 is a schematic view showing the timing sequence in the displaymode of the OLED pixel driving circuit provided by an embodiment of thepresent invention;

FIG. 5 is a schematic view showing the circuit connection in the sensemode of the OLED pixel driving circuit provided by an embodiment of thepresent invention;

FIG. 6 is a schematic view showing the timing sequence in the sense modeof the OLED pixel driving circuit provided by an embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

To further explain the technique means and effect of the presentinvention, the following uses preferred embodiments and drawings fordetailed description.

Refer to FIG. 2 to FIG. 6. The present invention provides an OLED pixeldriving circuit. As shown in FIG. 2, FIG. 3 and FIG. 5, the OLED pixeldriving circuit comprises: a first thin film transistor (TFT) T1, asecond TFT T2, a third TFT T3, a fourth TFT T4, a fifth TFT T5, acapacitor C1, an OLED D1, and a switch K, a digital-to-analog converter(DAC) DAC and an analog-to-digital converter (ADC) ADC disposed in adriving IC 10, wherein the fourth TFT T4 is the driving TFT directlydriving the OLED D1.

The OLED pixel driving circuit uses a 5T1C structure, and disposes aswitch K, wherein the operation states comprise a display mode and asense mode.

Specifically, the switch K is control by a switch signal Switch, andcomprises a first pin K1, a second pin K2, and a third pin K3.

The first TFT T1 has a gate connected to a scan signal Scan, a drainconnected to a power source OVDD, and a source connected to a drain ofthe second TFT T2, a gate g of the fourth TFT T4, and an end of thecapacitor C1. The second TFT T2 has a gate connected to the scan signalScan, and a source connected to a common ground voltage OVSS. The fourthTFT T4 has a drain connected to a power source voltage OVDD, and asource s connected to a drain of the fifth TFT T5. The fifth TFT T5 hasa gate connected to a light-emitting control signal EM, and a sourceconnected to an anode of the OLED D1. The OLED D1 has a cathodeconnected to the common ground voltage OVSS. The capacitor C1 has theother end connected to the source s of the fourth TFT T4. The third TFTT3 has a gate connected to the scan signal Scan in the display mode andconnected to the sensing control signal Sense in the sense mode, asource connected to the source s of the fourth TFT T4, and a drainconnected to the first pin K1 of the switch K.

The switch K has the second pin K2 connected to the DAC DAC, and thethird pin K3 connected to the ADC ADC.

The first TFT T1, the second TFT T2, the third TFT T3, and the fourthTFT T4 are all low temperature polysilicon (LTPS) TFts, oxidesemiconductor TFTs, or amorphous silicon (aSi) TFTs.

It should be noted that the resistance of the first TFT T1 and theresistance of the second TFT T2 have a specific ratio relation. When thefirst TFT T1 and the second TFT t2 are both turned on, the two TFTsperform voltage division on the power source voltage OVDD.

Refer to FIG. 3 and FIG. 4. In the display mode, the switch signalSwitch controls the switch K to connect the first pin K1 and the secondpin K2, and the DAC DAC provides the data signal Data. The display modeis further divided into a data writing stage t1 and a light-emittingstage t2.

In the data writing stage t1, the scan signal Scan is at high voltage sothat the first TFT T1, the second TFT T2, and the third TFT T3 are allturned on; the light-emitting control signal EM is at low voltage sothat the fifth TFT T5 is cut off. The common ground voltage OVSS is atlow voltage, and the data signal is at high voltage.

the conductive first TFT T1 and the second TFT T2 perform voltagedivision on the power source voltage OVDD so that the voltage Vg at thegate g of the fourth TFT T4 is:

Vg=OVDD×R _(T2)/(R _(T2) +R _(T1));

Wherein R_(T1) is the resistance of the first TFT T1, and R_(T2) is theresistance of the second TFT T2.

The data signal Data is written into the source s of the fourth TFT T4through the first pin K1 and the second pin K2 of the switch K and theconductive third TFT T3, i.e., Vs=V_(Data) (Vs is the voltage at thesource s of the fourth TFT T4, and V_(Data) is the voltage at the datasignal Data.)

Because in the data writing stage t1, the light-emitting control signalEM cuts off the fifth TFT T5, which cuts off the connection between theanode of the OLED D1 and the source s of the fourth TFT T4 so that theOLED D1 will not emit light. Hence, the OLED D1 is required to have aspecific higher threshold voltage to prevent the OLED D1 from emittinglight, and the light-emitting layer inside the OLED D1 can be of a2-layered, 3-layered, or 4-layered structure. The data signal Data isnot required to have a voltage not higher than the threshold voltage ofthe OLED D1, and the gate voltage Vg of the fourth TFT T4 has to be nothigher than the sum of the threshold voltage of the OLED D1 and thethreshold voltage of the fourth TFT T4. As such, the voltage of the datasignal Data and the gate voltage Vg of the fourth TFT T4 are notaffected by the threshold voltage of the OLED D1.

In the light-emitting stage t2, the scan signal Scan is at low voltageso that the first TFT T1, the second TFT T2, and the third TFT T3 areall cut-off; the light-emitting control signal EM is at high voltage sothat the fifth TFT T5 is turned on, and the anode of the OLED d1 isconnected to the source s of the fourth TFT T4. The common groundvoltage OVSS is at low voltage, and the data signal Data is at highvoltage. Under the storage effect of the capacitor C1, the OLED D1 emitslight to display.

It should be noted that the driving IC 10 presets a lowest grayscales tocorrespond to a highest voltage of the data signal Data, and a highestgrayscales to correspond to a lowest voltage of the data signal Data; byadjusting the ratio between the resistance of the first TFT T1 and theresistance of the second TFT T2, the gate of the fourth TFT T4 has avoltage Vg equal to the sum of the maximum of the data signal voltageVmax_(Data) and the threshold voltage Vth of the fourth TFT T4.

Vg=OVDD×R _(T2)/(R _(T2) +RT ₁)=Vmax_(Data) +Vth;

As such, the dark state display of the OLED display device has a betterdisplay quality.

Refer to FIG. 5 and FIG. 6. In the sense mode, the scan signal Scanfirst provides a high voltage pulse so that the first TFT T1 and thesecond TFT T2 are turned on. The conductive first TFT T1 and the secondTFT T2 perform voltage division on the power source voltage OVDD so thatthe voltage Vg at the gate g of the fourth TFT T4 is:

Vg=OVDD×R _(T2)/(R _(T2) +R _(T1)).

The light-emitting control signal EM stays always at low voltage to cutoff the fifth TFT T5, which cuts of the connection between the anode ofthe OLED D1 and the source s of the fourth TFT T4 (i.e., the drivingTFT) so that the OLED D1 does not emit light.

The common ground voltage OVSS always stays at low voltage.

The sensing control signal Sense first provides a high voltage pulsesynchronized with the high voltage pulse of the scan signal Scan to turnon the third TFT T3. The switch signal Switch maintains the switch K toconnect the first pin K1 and the second pin K2, the DAC first provides alow voltage signal written into the source s of the fourth TFT T4through the first pin K1 and the second pin K2 of the switch K and theconductive third TFT T3. Then, the switch signal Switch controls theswitch K to connect the first pin K1 and the third pin K3 of the switchK. At this point, because the he voltage Vg at the gate g of the fourthTFT T4 is Vg=OVDD×R_(T2)/(R_(T2)+R_(T1)), and the source s is at a lowervoltage, the fourth TFT T4 is conductive. The current flowing throughthe fourth TFT T4 enters the ADC ADC through the conductive third TFTT3, and the first pint K1 and the third pin K3 of the switch K, so thatthe ADC senses the threshold voltage Vth of the fourth TFT T4.

Then, the scan signal Scan and the sensing control signal Sense becomelow voltage and stay at low voltage.

After the ADC ADC senses the threshold voltage Vth of the fourth TFT T4(i.e., the driving TFT), the ADC ADC converts the threshold voltage Vthinto a digital sensing data and stores the digital sensing data forcompensation in the display mode. Since because the threshold voltageVth of the fourth TFT T4 (i.e., the driving TFT) in the display mode iscompensated, the current flowing through the OLED D1 is independent ofthe threshold voltage Vth of the driving TFT, and the impact of thethreshold voltage Vth of the driving TFT is eliminated, the displayuniformity and the light-emitting efficiency are improved.

Based on the same structure, the invention also provides an OLED displaydevice, comprising the aforementioned OLED pixel driving circuit. Thedetails will not be repeated here.

In summary, the present invention provides an OLED pixel driving circuitwith a 5T1C structure and using a switch. By the switch signalcontrolling the switch, the first pin and the second pin are connectedto enter the display mode, and by the switch signal controlling theswitch, the first pin and the third pin are connected to enter the sensemode, so that the ADC senses the threshold voltage of the fourth TFT,converted by ADC and used for data compensation in the display mode. Assuch, the invention is able to compensate and to eliminate the impact ofthe threshold voltage of the driving TFT on the current flowing throughthe OLED to achieve uniform display; the light-emitting control signalcontrols the fifth TFT connected to the OLED to cut off during the datawriting stage to keep the data signal voltage and the gate voltage ofthe driving TFT unaffected by the OLED threshold voltage. The inventionalso provides an OLED display device, comprising the OLED pixel drivingcircuit, able to compensate improve display uniformity and keep the datasignal voltage and the gate voltage of the driving TFT unaffected by theOLED threshold voltage.

It should be noted that in the present disclosure the terms, such as,first, second are only for distinguishing an entity or operation fromanother entity or operation, and does not imply any specific relation ororder between the entities or operations. Also, the terms “comprises”,“include”, and other similar variations, do not exclude the inclusion ofother non-listed elements. Without further restrictions, the expression“comprises a . . . ” does not exclude other identical elements frompresence besides the listed elements.

Embodiments of the present invention have been described, but notintending to impose any unduly constraint to the appended claims. Anymodification of equivalent structure or equivalent process madeaccording to the disclosure and drawings of the present invention, orany application thereof, directly or indirectly, to other related fieldsof technique, is considered encompassed in the scope of protectiondefined by the clams of the present invention.

What is claimed is:
 1. An organic light-emitting diode (OLED) pixeldriving circuit, comprising: a first thin film transistor (TFT), asecond TFT, a third TFT, a fourth TFT, a fifth, a capacitor, an OLED,and a switch, a digital-to-analog converter (DAC) and ananalog-to-digital converter (ADC) disposed in a driving integratedcircuit (IC), operation states of the OLED pixel driving circuitcomprising a display mode and a sense mode; the switch being controlledby a switch signal, comprising a first pin, a second pin, and a thirdpin; the first TFT having a gate connected to a scan signal, a drainconnected to a power source, and a source connected to a drain of thesecond TFT, a gate of the fourth TFT, and an end of the capacitor; thesecond TFT having a gate connected to the scan signal, and a sourceconnected to a common ground voltage; the fourth TFT having a drainconnected to a power source voltage, and a source connected to a drainof the fifth TFT; the fifth TFT having a gate connected to alight-emitting control signal and a source connected to an anode of theOLED; the OLED having a cathode connected to the common ground voltage;the capacitor having the other end connected to the source of the fourthTFT; the third TFT having a gate connected to the scan signal in thedisplay mode and connected to the sensing control signal in the sensemode, a source connected to the source of the fourth TFT, and a drainconnected to the first pin of the switch; the switch having the secondpin connected to the DAC, and the third pin connected to the ADC; in thedisplay mode, the switch signal controlling the switch to connect thefirst pin and the second pin, and the DAC providing the data signal; thedisplay mode being divided into a data writing stage and alight-emitting stage, and the light-emitting control signal is at lowvoltage to cut off the fifth TFT during the data writing stage, and athigh voltage to turn on the fifth TFT during the light-emitting stage;in the sense mode, the DAC first providing a low voltage signal, andthen the switch signal controlling the switch to connect the first pinand the third pin so that the ADC sensing a threshold voltage of thefourth TFT.
 2. The OLED pixel driving circuit as claimed in claim 1,wherein in the data writing stage of the display mode, the scan signalis at high voltage, the common ground voltage is at low voltage, and thedata signal is at high voltage; in the light-emitting stage of thedisplay mode, the scan signal is at low voltage, the common groundvoltage is at low voltage, and the data signal is at high voltage; inthe sense mode, the scan signal first provides a high voltage pulse andthen maintains at low voltage; the sensing control signal first providesa high voltage pulse synchronized with the high voltage pulse of thescan signal and then maintains at low voltage; the common ground voltagealways stays at low voltage; and the light-emitting control signalalways stays at low voltage.
 3. The OLED pixel driving circuit asclaimed in claim 2, wherein a resistance of the first TFT and aresistance of the second TFT have a ratio relation.
 4. The OLED pixeldriving circuit as claimed in claim 3, wherein the driving IC presets alowest grayscales to correspond to a highest voltage of the data signal,and a highest grayscales to correspond to a lowest voltage of the datasignal; by adjusting the ratio between the resistance of the first TFTand the resistance of the second TFT, the gate of the fourth TFT has avoltage equal to the sum of the maximum of the data signal voltage andthe threshold voltage of the fourth TFT.
 5. An organic light-emittingdiode (OLED) display device, comprising an OLED pixel driving circuit;the OLED pixel driving circuit further comprising: a first thin filmtransistor (TFT), a second TFT, a third TFT, a fourth TFT, a fifth, acapacitor, an OLED, and a switch, a digital-to-analog converter (DAC)and an analog-to-digital converter (ADC) disposed in a drivingintegrated circuit (IC), operation states of the OLED pixel drivingcircuit comprising a display mode and a sense mode; the switch beingcontrolled by a switch signal, comprising a first pin, a second pin, anda third pin; the first TFT having a gate connected to a scan signal, adrain connected to a power source, and a source connected to a drain ofthe second TFT, a gate of the fourth TFT, and an end of the capacitor;the second TFT having a gate connected to the scan signal, and a sourceconnected to a common ground voltage; the fourth TFT having a drainconnected to a power source voltage, and a source connected to a drainof the fifth TFT; the fifth TFT having a gate connected to alight-emitting control signal and a source connected to an anode of theOLED; the OLED having a cathode connected to the common ground voltage;the capacitor having the other end connected to the source of the fourthTFT; the third TFT having a gate connected to the scan signal in thedisplay mode and connected to the sensing control signal in the sensemode, a source connected to the source of the fourth TFT, and a drainconnected to the first pin of the switch; the switch having the secondpin connected to the DAC, and the third pin connected to the ADC; in thedisplay mode, the switch signal controlling the switch to connect thefirst pin and the second pin, and the DAC providing the data signal; thedisplay mode being divided into a data writing stage and alight-emitting stage, and the light-emitting control signal is at lowvoltage to cut off the fifth TFT during the data writing stage, and athigh voltage to turn on the fifth TFT during the light-emitting stage;in the sense mode, the DAC first providing a low voltage signal, andthen the switch signal controlling the switch to connect the first pinand the third pin so that the ADC sensing a threshold voltage of thefourth TFT.
 6. The OLED display device as claimed in claim 5, wherein inthe data writing stage of the display mode, the scan signal is at highvoltage, the common ground voltage is at low voltage, and the datasignal is at high voltage; in the light-emitting stage of the displaymode, the scan signal is at low voltage, the common ground voltage is atlow voltage, and the data signal is at high voltage; in the sense mode,the scan signal first provides a high voltage pulse and then maintainsat low voltage; the sensing control signal first provides a high voltagepulse synchronized with the high voltage pulse of the scan signal andthen maintains at low voltage; the common ground voltage always stays atlow voltage; and the light-emitting control signal always stays at lowvoltage.
 7. The OLED display device as claimed in claim 6, wherein aresistance of the first TFT and a resistance of the second TFT have aratio relation.
 8. The OLED display device as claimed in claim 7,wherein the driving IC presets a lowest grayscales to correspond to ahighest voltage of the data signal, and a highest grayscales tocorrespond to a lowest voltage of the data signal; by adjusting theratio between the resistance of the first TFT and the resistance of thesecond TFT, the gate of the fourth TFT has a voltage equal to the sum ofthe maximum of the data signal voltage and the threshold voltage of thefourth TFT.
 9. An organic light-emitting diode (OLED) pixel drivingcircuit, comprising: a first thin film transistor (TFT), a second TFT, athird TFT, a fourth TFT, a fifth, a capacitor, an OLED, and a switch, adigital-to-analog converter (DAC) and an analog-to-digital converter(ADC) disposed in a driving integrated circuit (IC), operation states ofthe OLED pixel driving circuit comprising a display mode and a sensemode; the switch being controlled by a switch signal, comprising a firstpin, a second pin, and a third pin; the first TFT having a gateconnected to a scan signal, a drain connected to a power source, and asource connected to a drain of the second TFT, a gate of the fourth TFT,and an end of the capacitor; the second TFT having a gate connected tothe scan signal, and a source connected to a common ground voltage; thefourth TFT having a drain connected to a power source voltage, and asource connected to a drain of the fifth TFT; the fifth TFT having agate connected to a light-emitting control signal and a source connectedto an anode of the OLED; the OLED having a cathode connected to thecommon ground voltage; the capacitor having the other end connected tothe source of the fourth TFT; the third TFT having a gate connected tothe scan signal in the display mode and connected to the sensing controlsignal in the sense mode, a source connected to the source of the fourthTFT, and a drain connected to the first pin of the switch; the switchhaving the second pin connected to the DAC, and the third pin connectedto the ADC; in the display mode, the switch signal controlling theswitch to connect the first pin and the second pin, and the DACproviding the data signal; the display mode being divided into a datawriting stage and a light-emitting stage, and the light-emitting controlsignal is at low voltage to cut off the fifth TFT during the datawriting stage, and at high voltage to turn on the fifth TFT during thelight-emitting stage; in the sense mode, the DAC first providing a lowvoltage signal, and then the switch signal controlling the switch toconnect the first pin and the third pin so that the ADC sensing athreshold voltage of the fourth TFT; wherein in the data writing stageof the display mode, the scan signal being at high voltage, the commonground voltage being at low voltage, and the data signal being at highvoltage; in the light-emitting stage of the display mode, the scansignal being at low voltage, the common ground voltage being at lowvoltage, and the data signal being at high voltage; in the sense mode,the scan signal first providing a high voltage pulse and thenmaintaining at low voltage; the sensing control signal first providing ahigh voltage pulse synchronized with the high voltage pulse of the scansignal and then maintaining at low voltage; the common ground voltagealways staying at low voltage; and the light-emitting control signalalways staying at low voltage; wherein a resistance of the first TFT anda resistance of the second TFT having a ratio relation; wherein thedriving IC presetting a lowest grayscales to correspond to a highestvoltage of the data signal, and a highest grayscales to correspond to alowest voltage of the data signal; by adjusting the ratio between theresistance of the first TFT and the resistance of the second TFT, thegate of the fourth TFT having a voltage equal to the sum of the maximumof the data signal voltage and the threshold voltage of the fourth TFT.